In the '70s, effect manufacturers introduced phase pedals attempting to recreate the vibey tape flanging sounds heard on psychedelic records. Unlike flangers—which use delay to emulate this sound—phasers use an LFO wave sent through all-pass filters to produce their chewy, modulated sound. Ultimately, these pedals add a totally different and new palette of sounds to explore. Since then, phase shifters have been an essential modulation sound on many guitarists' pedalboards.
The Boss PH-3 Phase Shifter provides a wealth of vintage and modern phasing effects. Via the "Stage" control, you're able to select 4, 8, 10, and 12-stage phasers, plus new "Rise" and "Fall" effects for unidirectional phasing. You can control the rate with the rate knob or add an expression pedal to control the rate in real time. It even adds a tap tempo function, so you can sync the rate to your own tempo.
From there, the PH-3 gives you two additional controls "Depth" and "Res." Depth controls the amplitude of the modulation, i.e. the size of the range in which the effect sweeps. Res sets the amount of effected signal sent back through the effect, so as you turn the knob higher, the overall effect level gets bigger and stronger. In combination with the "Stage" control, there's a ton of versatility, and the different options make it easy to set the rate to your liking.
This 2009 Boss PH-3 shows some cosmetic signs of wear (it does get stepped on after all!), but it sounds great and functions 100% as it should.